1. Field of the Invention
The present disclosure relates to an organic light emitting diode (OLED) display device, and more particularly, to a method of fabricating an organic light emitting diode (OLED) display device where deterioration of a shadow mask is prevented.
2. Discussion of the Related Art
Among various flat panel display devices (FPDs), an organic light emitting diode (OLED) display device has a relatively high brightness and a relatively low driving voltage. In addition, since the OLED display device has an emissive type emitting a light for itself, the OLED display device has a relatively high contrast ratio and a relatively thin profile. The OLED display device has an advantage in displaying moving images due to a response time of several microseconds. Further, the OLED display device has no limitation in a viewing angle and has stability even at a low temperature. Since the OLED display device is driven with a low voltage of direct current (DC) 5V to DC 15V, it is easy to design and fabricate a driving circuit. Moreover, since a deposition apparatus and an encapsulation apparatus are all that is needed for fabricating the OLED display device, the fabrication process for the OLED display device is very simple.
FIG. 1 is a plan view showing an organic light emitting diode display device according to the related art. In FIG. 1, an organic light emitting diode (OLED) display device 10 according to the related art includes first, second and third sub-pixels SP1, SP2 and SP3 arranged in a stripe type. The first sub-pixels SP1 emitting a red colored light are arranged in a first stripe S1 along a vertical direction, the second sub-pixels SP2 emitting a green colored light are arranged in a second stripe S2 along the vertical direction, and the third sub-pixels SP3 emitting a blue-colored light are arranged in a third stripe S3 along the vertical direction. In addition, the first, second and third sub-pixels SP1, SP2 and SP3 are sequentially arranged along a horizontal direction. The first, second and third sub-pixels SP1, SP2 and SP3 constitute a single pixel P.
An organic luminescent layer is formed in each of the first, second and third sub-pixels SP1, SP2 and SP3 by depositing an organic luminescent material using a shadow mask having an opening and a rib surrounding the opening. The organic luminescent materials in the first, second and third sub-pixels SP1, SP2 and SP3 are divided by the shadow mask and the luminescent property of the organic luminescent material is improved by the shadow mask.
The OLED display device is classified into a top emission type and a bottom emission type according to an emission direction of light. Since an aperture ratio of the top emission type OLED display device is greater than an aperture ratio of the bottom emission type OLED display device, the top emission type OLED display device has been widely used. However, since the shadow mask for the top emission type OLED display device has the opening having a relatively great area and the rib having a relatively narrow width. As a result, when the organic luminescent material is deposited by using the shadow mask, the shadow mask is deteriorated. For example, the opening of the shadow mask is blocked by the organic luminescent material such that the area of the opening is reduced and the adjacent ribs contact each other.
The organic luminescent layer may be formed as a multiple layer including a hole injecting layer, a hole transporting layer, an emitting material layer and an electron transporting layer. Specifically, the hole transporting layers of the first, second and third sub-pixels have different thicknesses from each other.
FIG. 2 is a cross-sectional view showing an organic light emitting diode display device according to the related art. In FIG. 2, the organic light emitting diode display device 10 according to the related art includes the pixel P, and the pixel P includes the first, second and third sub-pixels SP1, SP2 and SP3 corresponding to red, green and blue colors, respectively. An organic light emitting diode in each of the first, second and third sub-pixels SP1, SP2 and SP3 includes a first electrode 11, a hole injecting layer (HIL) 13, a hole transporting layer (HTL) 16, an emitting material layer (EML) 24, an electron transporting layer (ETL) 30 and a second electrode 35.
The hole transporting layer 16 includes first, second and third hole transporting layers 16a, 16b and 16c, and the emitting material layer 24 includes first, second and third emitting material layers 24a, 24b and 24c emitting red, green and blue colored lights, respectively. The first hole transporting layer 16a is formed in the entire pixel P with an equal thickness. The second and third hole transporting layers 16b and 16c are formed in the first and second sub-pixels SP1 and SP2, respectively, with different thicknesses. As a result, the first and second hole transporting layers 16a and 16b of the first sub-pixel SP1, the first and third hole transporting layers 16a and 16c of the second sub-pixel SP2 and the first hole transporting layer 16a of the third sub-pixel SP3 have different thicknesses from each other.
The first, second and third emitting material layers 24a, 24b and 24c have different luminous efficiencies from each other. The thickness of the hole transporting layer 16 in each of the first, second and third sub-pixels SP1, SP2 and SP3 is determined based on an optical thickness for maximizing a micro cavity effect according to the luminous efficiency of each of the first, second and third emitting material layers 24a, 24b and 24c. 
The hole injecting layer 13 and the first hole transporting layer 16a are sequentially formed on the first electrode 11 by using a first shadow mask having a first opening corresponding to the entire pixel P. Next, the third hole transporting layer 16c is formed on the first hole transporting layer 16a by using a second shadow mask having a second opening corresponding to the second sub-pixel SP2, and the second hole transporting layer 16b is formed on the first hole transporting layer 16a by using a third shadow mask having a third opening corresponding to the first sub-pixel SP1. Next, the third emitting material layers 24c is formed on the first hole transporting layer 16a by using a fourth shadow mask having a fourth opening corresponding to the third sub-pixel SP3, and the second emitting material layers 24b is formed on the third hole transporting layer 16c by using a fifth shadow mask having a fifth opening corresponding to the second sub-pixel SP2. In addition, the first emitting material layers 24a is formed on the second hole transporting layer 16b by using a sixth shadow mask having a sixth opening corresponding to the first sub-pixel SP1. Next, the electron transporting layer 30 is formed on the first, second and third emitting material layers 24a, 24b and 24c by using a seventh shadow mask having a seventh opening corresponding to the entire pixel P, and the second electrode 35 is formed on the electron transporting layer 30.
In the organic light emitting diode display device according to the related art, since the second and third hole transporting layers 16b and 16c are further formed in the first and second sub-pixels SP1 and SP2, respectively, the more organic luminescent material is used for the organic luminescent layer and the material cost increases. In addition, since the organic luminescent material has different heat capacities, different melting points and different boiling points, the organic luminescent material may remains on the shadow mask to reduce the area of the opening such that the adjacent ribs contact each other after the organic luminescent material is deposited by using the shadow mask. The blocking of the opening (i.e. the rib contact) of the shadow mask causes deterioration of patterns in a subsequent depositing step.
FIG. 3 is a plan view showing a shadow mask used for forming a sub-pixel of an organic light emitting diode display device according to the related art, and FIG. 4 is a phase diagram for an organic luminescent material used for an organic light emitting diode display device according to the related art. In FIG. 3, a shadow mask 50 includes an opening OP of a rectangular shape and a rib RB surrounding the opening OP before the shadow mask 50 is used for depositing an organic luminescent material. However, after the shadow mask 50 is used for depositing an organic luminescent material several times to form the second and third hole transporting layers 16b and 16c (of FIG. 2), the organic luminescent material remains on the shadow mask 50 and the opening OP is blocked by the residual organic luminescent material such that the adjacent rib RB contact each other.
The rib contact deterioration of the shadow mask 50 relates to physical properties such as a heat capacity, a melting point and a boiling point in a material point of view. In a vacuum thermal evaporation process, molecules of the organic luminescent material are evaporated to be a gas state and are deposited on a substrate. In FIG. 4, the organic luminescent material transitions along an arrow (A) from a solid state to a gas state through a liquid state under a first process pressure P1, and the organic luminescent material of the gas state is deposited on the substrate. Before the gas state of the molecules of the organic luminescent material becomes a complete inactive state (i.e. the solid state) on the substrate, the molecules of the organic luminescent material have an active state (i.e. the liquid state) as an intermediate state for a temperature period TPA. Since the molecules of an active state have an attractive force therebetween due to a physical reaction instead of a chemical reaction, a tensile force the molecules of an active state increases so that the molecules of an active state can perform a gelatination.
Since the adjacent ribs RB of the shadow mask 50 are relatively close to each other, a portion of the molecules of an active state adheres to the rib RB due to a physical reaction while the shadow mask 50 is aligned to the substrate. As the vacuum thermal evaporation process is repeated, an amount of the molecules adhering to the rib RB increases so that the adjacent ribs RB can contact each other.